I believe my lapping plate and my technique are both ok. I don't get this bulge-pattern on my GlassStones.

I let the Naniwa dry out for a day and now it has a slight hollow in the middle where the bulge was. So I'm becoming more convinced it is related to wetting and drying, perhaps aggravated by the fact that it is a thick and porous stone. I think I will start lapping it as dry as possible and use it with as little water as possible. Maybe that way it will maintain a more stable surface.